Position detection using a spaced apart array of magnetic field generators and plural sensing loop circuits offset from one another in the measurement direction

ABSTRACT

A sensor head is provided for use in a apparatus for indicating the position of a movable member relative to a fixed member. It has two multi-turn sensor windings, each including repetitive pattern of series connected alternate sense loops of conductor. One sensor winding is in spatial phase quadrature with the other sensor winding, and the arrangement of the windings is such that the mid points of the sensor windings coincide. The sensor head is particularly suited for use in indicating the position of a movable member relative to a fixed member as it is relatively insensitive to longitudinal tilt relative to the other member.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to position encoders generally. Theinvention has particular although not exclusive relevance to non-contactlinear position encoders.

2. Related Art

Many types of non-contact linear position sensors have been proposed. Inparticular, EP 0182085 discloses a non-contact position sensor whichemploys an excitation winding and one or more pick-up windings mountedon the surface of a stationary element, and a conductive screen mountedon a movable element. A homogenous a.c. magnetic field is established inthe vicinity of the pick-up winding by passing a current through agenerally planar excitation winding. The pick-up winding, normallyconsisting of one turn, starts at one end of the stationary element andfollows a sinuous path therealong until it reaches the other end, whereit turns back along the support following a sinuous path to the startingend. The sinusoidal forward and return conduction paths that form thepick-up winding are substantially 180° out of phase. Therefore, thepick-up winding comprises a series of alternating sense conductionloops, each arranged to enclose a similar area.

If the area enclosed by each loop of the pick-up winding is identical,and there is a homogenous excitation drive field over the length of thepick-up winding then, in the absence of the conductive screen, therewill be no net output from the pick-up winding. However, when theconductive screen is provided adjacent the pick-up winding thehomogenous field generated by the current flowing in the excitationwinding induces eddy currents in the conductive screen. These eddycurrents establish a counter-field opposing the forward homogenousfield. This opposing field alters the balance between the excitationwinding and the pick-up winding and a net output EMF in the pick-upwinding results, the magnitude of which is dependent upon the positionof the conductive screen within a period of the pick-up winding. Inparticular, the peak amplitude of the output signal from the pick-upwinding varies in a sinusoidal manner with the position of theconductive screen along the pick-up winding.

In order to determine the position of the conductive screen within awhole period of the pick-up winding, a second pick-up winding isprovided which is in spatial phase quadrature with the first pick-upwinding. With this arrangement two phase quadrature signals aregenerated, from which the position of the conductive screen within aperiod of the pick-up winding can be determined, independent of theamplitudes of the signals. Additionally, if the absolute position of theconductive screen is to be determined, then either a counter must beprovided for counting the number of periods that have passed from areference point or an additional coarse position encoder must beprovided.

The present applicant has proposed in International ApplicationWO95/01095 a similar position sensor, which employs a resonant circuitinstead of the conductive screen. By using a resonant circuit the outputsignal levels are increased and the system can be operated in apulse-echo mode of operation, i.e. applying a short burst of excitationcurrent to the excitation winding and then detecting and processing thesignal induced in the pick-up windings, after the burst of excitationcurrent has ended. Pulse-echo operation is possible because the resonantcircuit continues to "ring" for a short period of time after theexcitation current has been removed. This offers the advantage ofensuring that there is no unwanted cross-coupling between the excitationwinding and the pick-up windings.

Although use of a resonant circuit in the position sensor allows apulse-echo mode of operation, this is not essential. When the resonantcircuit is resonating, its impedance is purely resistive. Consequently,the electrical phase of the output signal with respect to the drivevoltage is well defined, and the desired output signal can be isolatedfrom any unwanted cross-coupling signal by synchronously detecting thesignals on the pick-up windings at the appropriate phase. In contrast,when a conductive screen is used, the eddy currents induced in theconductive screen will include a resistive component and an inductivecomponent which may be difficult to define.

A problem with the position sensors described in EP 0182085 andWO95/01095 is that when there is a large measuring distance and when ahigh resolution of position measurement is required, a large excitationloop must be energised. For example, if the measurement range is 50meters then the area enclosed by the excitation winding must be 50meters long to enable the system to work properly. Energising thisamount of area results in a large amount of radiated interference.Additionally, the longer pick-up windings are more sensitive to unwantedelectromagnetic interference.

U.S. Pat. No. 4,820,961 solves the above problem by using a passivestrip of spaced conductive shields mounted on the stationary element,and a sensor head, comprising the excitation winding and the pick-upwindings, mounted on the movable element. In particular, U.S. Pat. No.4,820,961 discloses a non-contact linear position sensor for determiningthe position of a moveable vehicle along a fixed track. Along the trackthere are a plurality of equally spaced conductive shields, and driveand pick-up windings are provided on the vehicle. As the vehicle movesalong adjacent the track, output signals are induced in the pick-upwindings from which the position of the vehicle can be determined.

However, the system disclosed in U.S. Pat. No. 4,820,961 is not suitablefor more accurate position sensing systems, as for example positionsensing systems used in the positioning of machine tools, where positionsensing accuracy is typically required to be better than 20 μm, becausethe system is relatively sensitive to pitch and roll of the sensor headrelative to the track.

SUMMARY OF THE INVENTION

According to one aspect, the present invention provides a sensor headfor use in an apparatus for indicating the position of a movable memberrelative to a fixed member, the sensor head comprising: at least twosensor windings, mounted on the sensor head, each comprising at leastone period of series connected alternate sense loops of conductor,wherein each sensor winding is spatially separated in the measurementdirection and wherein the arrangement of the multi-turn sensor windingsis such that their respective mid-points substantially coincide. Such aconfiguration of sensor head is advantageous in that it is lesssensitive to longitudinal tilt of the sensor head relative to the othermember.

Each sensor winding can be defined by conductors on a plurality oflayers of a printed circuit board. This has the advantage of reducedmanufacturing cost. Preferably, the sensor windings are defined by twocomplementary repetitive patterns of conductors mounted on two sides ofthe printed circuit board, and wherein each side of the printed circuitboard carries a portion of each repetitive pattern. With thisarrangement, the sensitivity of the position encoder to roll of thesensor head relative to the fixed track is reduced.

An embodiment of the present invention provides an apparatus forindicating the position of a movable member relative to a fixed member,comprising: a plurality of magnetic field responsive elements equallyspaced along the fixed member; and a sensor head as described above,mounted for movement with the movable member, and arranged such thatwhen the magnetic field responsive elements are energised, signals areinduced in the sensor windings which are dependent upon the position ofthe movable member relative to the fixed member. Preferably, themagnetic field responsive elements are resonant circuits, as this allowsa pulse-echo mode of operation.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the present invention will now be described,with reference to the accompanying drawings in which:

FIG. 1 is a schematic view of a linear position sensor suggested in theprior art;

FIG. 2a schematically shows part of a track and part of a sensor headwhich are used in the position encoder shown in FIG. 1;

FIG. 2b shows the way in which a demodulated output signal from apick-up winding mounted on the sensor head varies as a function of theposition of the sensor head relative to the track;

FIG. 3 schematically illustrates the track and, on the left hand side, asensor head in an ideal position lying in a plane parallel to the planeof the track and, on the right hand side, a sensor head which is tiltedrelative to the track;

FIG. 4a schematically illustrates a sensor head embodying the presentinvention having two phase quadrature pick-up windings, which compriseseries connected alternate sense hexagonal shaped loops;

FIG. 4b shows a first layer of printed conductors which forms part ofthe sensor head shown in FIG. 4a;

FIG. 4c shows a second layer of printed conductors which forms part ofthe sensor head shown in FIG. 4a;

FIG. 5a schematically illustrates the form of a first winding formingpart of the sensor head shown in FIG. 4a;

FIG. 5b schematically illustrates the form of a second winding formingpart of the sensor head shown in FIG. 4a which is in spatial phasequadrature with the winding shown in FIG. 5a;

FIG. 6 is a plot of the surface current density on some of theconductors of one of the windings on the sensor head shown in FIG. 4a;

FIG. 7a schematically illustrates the pattern of conductors forming afirst part of the sensor head shown in FIG. 4a;

FIG. 7b schematically illustrates the pattern of conductors forming asecond part of the sensor head shown in FIG. 4a;

FIG. 7c schematically illustrates the pattern of conductors forming athird part of the sensor head shown in FIG. 4a;

FIG. 7d schematically illustrates the pattern of conductors forming afinal part of the sensor head shown in FIG. 4a;

FIG. 8a schematically represents two sensor windings having differentmid points;

FIG. 8b schematically represents a modification to one of the sensorwindings shown in FIG. 8a which makes both windings have the same midpoint;

FIG. 8c schematically shows a modification to both windings shown inFIG. 8b so that both windings are balanced and enclose a similar area;

FIG. 9a illustrates a first layer of printed conductors which forms partof a sensor head according to a second embodiment;

FIG. 9b shows a second layer of printed conductors which forms part ofthe sensor head according to the second embodiment, which whensuperimposed on or under the layer shown in FIG. 9a, forms a preferredform of sensor head similar to the sensor head shown in FIG. 4a;

FIG. 10 is a block diagram illustrating the components of processingcircuitry used in one embodiment to determine the position of the sensorhead relative to the track;

FIG. 11 shows the end of a track used in a positional encoder of analternative embodiment, where the conductive screens have a rectangularslit provided in a central portion thereof;

FIG. 12a schematically represents the surface current density flowing inthe vertical limbs of two of the screens shown in FIG. 11;

FIG. 12b is a plot of the magnetic field generated at the surface of theconductive screens shown in FIG. 12a;

FIG. 13a shows the end of a track used in a positional encoder of apreferred embodiment, where the conductive screens are replaced byresonant circuits;

FIGS. 13b and 13c are layers of printed conductors which form the coilof the resonant circuits shown in FIG. 13a; and

FIG. 14 shows the end of a track used in a positional encoder of analternative embodiment, where the conductive screens are replaced byshort circuit coils.

BRIEF DESCRIPTION OF EXEMPLARY EMBODIMENTS

FIG. 1 is a schematic view of a linear position sensor suggested in theprior art. As shown in FIG. 1, the track 101 carries a plurality ofequally spaced conductive screens 103 made from, for example, copper.FIG. 1 also shows a pair conductive pick-up windings 105 and 107 inspatial phase quadrature and an excitation winding 109 mounted on asensor head 111. For best results, the height H of the conductivescreens 103 should be greater than the peak width W₁ of the pick-upwindings 105 and 107. As represented by arrow 113, the sensor head 111is free to move along the length of the track 101, i.e. parallel to thex-axis of FIG. 1. The track 101 is arranged to lie in a plane that isparallel to the plane in which the sensor head 111 can move, as thisprovides the greatest magnetic coupling between the pick-up windings 105and 107 and the conductive screens 103.

The pick-up windings 105 and 107 are formed as conductive patterns on aprinted circuit board, and are insulated from each other by using bothsides of the printed circuit board and via holes. Pick-up winding 105extends from point A at end 111a of the sensor head 111 and follows asinuous path therealong until it reaches point B at the other end 111b,where it returns back along the sensor head 111 following a sinuous pathto the starting point A. Similarly, pick-up winding 107 extends frompoint C at end 111a of the sensor head 111 and follows a sinuous paththerealong until it reaches point D at the other end 111b, where itreturns back along the sensor head 111 following a sinuous path to thestarting point C. The sinusoidal forward and return conduction pathsthat form each pick-up winding 105 and 107 are substantially 180° out ofphase. Provided each winding 105 and 107 extends along the sensor head111 for a whole number of periods T_(s), then each winding 105 and 107will be relatively insensitve to background electromagneticinterference. This is because each winding 105 and 107 comprises anequal number of alternate sense loops, ie. an equal number of loopswound clockwise and anticlockwise. Therefore any EMF induced in loopswound clockwise from background electromagnetic interference will cancelwith the EMF induced in the loops which are wound anticlockwise.Therefore, the windings 105 and 107 are said to be balanced.

The spatial period of the pick-up windings 105 and 107 and therepetition period of the conductive screens 103 should be substantiallythe same, so that the position of each screen within a period of apick-up winding is substantially the same. Consequently, the signalsinduced in the pick-up windings 105 and 107 by each conductive screen103 will all depend upon the same position and will add to give astronger output signal.

The configuration of the excitation winding 109 is designed to generate,upon excitation, a homogenous magnetic field along the x-axis for afixed sensor head position in the Z and Y planes. The excitation winding109 starts at end 111a of the sensor head 111 and extends around theperiphery of the sensor head 111 until it returns to end 111a. The ends115,117,119 and 121 of the pick-up windings 105 and 107 and the ends 123and 125 of the excitation winding 109 are connected to an excitation andprocessing unit (not shown), which produces the excitation signal andprocesses the signals on the pickup windings to determine the positionof the sensor head 111 relative to the track 101.

The operation of the sensor system shown in FIG. 1 will now be brieflydescribed with reference to FIGS. 2a and 2b. FIG. 2a shows three of theconductive screens 103 which are mounted on the track 101 and a portionof one of the pick-up windings 107. FIG. 2b shows the way in which thedemodulated output signal on pick-up winding 107 varies, as a functionof the position (x) of the sensor head 111 along the track 101, when anexcitation current is applied to the excitation winding (109 shown inFIG. 1). The maximum demodulated output signal occurs when the centresof the conductive screens 103 coincide with the largest separation ofthe forward and return conductor paths of winding 107, and the minimumoccurs when the centres of the conductive screens 103 coincide with thecrossover points of the forward and return conduction paths of winding107.

When the position of the sensor head 111 along the x-axis relative tothe track 101 is to be determined, an excitation current is applied tothe excitation loop (109 shown in FIG. 1). The excitation currentinduces eddy currents to flow within the conductive screens 103 whichare adjacent to the excitation winding. The induced eddy currentsestablish a counter-field opposing the excitation field. Thiscounter-field is sensed by the pick-up windings 105 and 107, and phasequadrature output signals are generated whose peak amplitudes vary (asshown in FIG. 2b) in a sinusoidal manner as the sensor head 111 movesalong the x-axis relative to the track 101. Therefore, by taking thearc-tangent of the ratio of the signals induced in the pick-up windings105 and 107, the position of the sensor head 111 within the repetitionperiod of the conductive screens 103 can be determined. To determine theabsolute position of the sensor head 111 along the entire length of thetrack 101, a counter is provided in the excitation and processingcircuitry (not shown) which counts the passing conductive screens 103. Amore detailed explanation of the way in which a system similar to theone shown in FIG. 1 operates, can be found in EP 0182085 and U.S. Pat.No. 4,820,961 the contents of which are incorporated herein byreference.

The inventors have identified a problem with the non-contact positionsensor illustrated in FIG. 1 which makes it unsuitable for high accuracyapplications, such as machine tool applications which require anaccuracy of better than 20 μm. In particular, the inventors have notedthat if the sensor head 111 tilts in the X-Y plane of FIG. 1, then apositional error occurs in the output signals. The reason for this errorwill now be described with reference to FIGS. 1 and 3.

FIG. 3 schematically illustrates the track 101 and, on the left handside, a sensor head 111 in an ideal position lying in a plane parallelto the plane of the track 103 and, on the right hand side, a sensor head111' which is tilted relative to the track 101. When the sensor head 111lies in a plane parallel to the plane of the track 101, the separation Sbetween the pick-up windings mounted on the sensor head 111 and theconductive screens mounted on the track 101 will be the same for allpoints along the sensor head 111. However, when the sensor head 111' istilted relative to the track 101, as shown, the separation S₁ betweenend 111'a of the sensor head 111' and the track 101 is smaller than theseparation S₂ between end 111b of the sensor head 111' and the track101. Consequently, those parts of the pick-up windings 105 and 107 whichare closer to the track 101 will pick-up more signal than those parts ofthe pick-up windings 105 and 107 which are further away.

Referring to FIG. 1, since the pick-up windings 105 and 107 arespatially separated by a quarter of a period T_(s) their mid-points willlikewise be separated by quarter of a period. As a result, when thesensor head 111' is tilted, for example about the mid-point of pick-upwinding 105, half of winding 105 will be closer to the track and halfwill be further away, whereas less than half of winding 107 will becloser to the track and more than half will be further away. Therefore,the output signals from each winding 105 and 107 will be affected in aslightly different manner. Consequently, when the processing circuitryperforms the ratio-metric calculation to determine the position of thesensor head 111 relative to the track 101 a positional error occurs.

The present embodiment aims to reduce this positional error whichresults from tilt of the sensor head 111 relative to the track 101, byusing pick-up windings whose effective mid points coincide. Preferablyeach winding is generally symmetric about the mid point, such that thewindings longitudinally to the left of the mid point are substantially amirror image of the windings longitudinally to the right of the midpoint.

FIG. 4a schematically illustrates a sensor head 111 having pick-upwindings 133 and 135, each comprising five periods of series connectedalternate sense hexagonal shaped loops of conductor. In this embodiment,the period (T_(s)) is equal to 6 mm and winding 135 is quarter of aperiod out of phase with winding 133. However, as will be described inmore detail below, the windings 133 and 135 have the same mid points(represented by the cross in the centre of the windings) on the sensorhead 111. The pick-up windings 133 and 135 are formed by conductivepatterns on two sides of a printed circuit board. The conductors on thetop layer are represented by unbroken lines, whereas those on the bottomlayer are represented by broken lines. The conductors on both sides ofthe printed circuit board are connected where appropriate at via holes1-46,1'-40',20a and 22a. In this embodiment, the connection points forconnecting the pick-up windings 133 and 135 to the excitation andprocessing circuitry (not shown) are provided at vias 20 and 21 forpick-up winding 133 and 22 and 23 for pick-up winding 135.

As shown in FIG. 4a, in this embodiment, the excitation winding 137extends around the periphery of the circuit board from the connectionpoint 123 in a decreasing clockwise spiral for four turns until via holeX, where it passes to the other side of the circuit board and thenextends clockwise in an increasing spiral for four turns to theconnection point 125. The connection points 123 and 125 are provided forconnecting the excitation winding 137 to the excitation and processingcircuitry (not shown).

FIG. 4b shows the conductive patterns and the via holes on the top sideof the printed circuit board, and FIG. 4c shows the conductive patternsand the via holes on the bottom of the printed circuit board (as viewedfrom the top side) which form the sensor head shown in FIG. 4a.

FIGS. 5a and 5b illustrate the form of each of the two pick-up windings133 and 135 respectively. As shown in FIG. 5a, winding 133 comprises anumber of hexagonally shaped loops of series connected conductors,connected such that adjacent loops are wound in the opposite sense.Pick-up winding 133 is such that two turns of conductors are providedfor each loop except for the loops at each end, which only have oneturn. In this embodiment, the repetition period of the loops is matchedwith the repetition period of the conductive screens so that theposition of each screen within a period of pick-up winding 133 issubstantially the same.

As shown in FIG. 5b, like pick-up winding 133, pick-up winding 135 alsocomprises a number of hexagonally shaped loops of series connectedconductors, connected such that adjacent loops are wound in the oppositesense. As shown, pick-up winding 135 has two turns of conductor per loopand has the same total number of hexagonal shaped loops as pick-upwinding 133. As shown in FIG. 5a and 5b, the mid-point (represented bythe cross) of pick-up winding 133 is substantially the same as themid-point of pick-up winding 135. Therefore, if the sensor head 111tilts relative to the track 101, then the same proportion of eachwinding will be closer to the track and the same proportion of eachwinding will be further away from the track. Consequently, the signalsoutput by each pick-up winding 133 and 135 will experience a similaramplitude change, which will be cancelled out by the ratio-metriccalculation performed by the excitation and processing circuitry (notshown).

As can be seen from FIGS. 5a and 5b, pick-up winding 133 extends over agreater distance than pick-up winding 135. In order to compensate forthis, the hexagonal shaped loop at each end of pick-up winding 133 ismade less sensitive to magnetic field than the other loops of pick-upwinding 133. In this embodiment, this is achieved by using only a singleturn of conductor to define the end loops.

In this embodiment, the separation d₁ is made to be approximately halfthe separation d₂. This is to make the pick-up windings 133 and 135 lesssensitive to some of the higher order harmonics of the opposing fieldcreated by the conductive screens 103 which are energised. This resultsfrom the current density which is induced in the pick-up windings by theopposing magnetic field.

FIG. 6 shows a plot of the current density (J) flowing in the verticallimbs of one period of one of the pick-up windings shown in FIG. 5. Froma Fourier analysis of the current density it can be shown that thiscurrent density can be generated by a fundamental having a period T_(s)and higher order odd harmonics (the even harmonics are zero because ofthe symmetry). It can also be shown that with d₂ =2d₁, the thirdharmonic content of the current density is approximately zero. As aresult, the pick-up windings 133 and 135 are sensitive to magneticfields which vary periodically along the length of the sensor head witha period T_(s), but they are relatively insensitive to magnetic fieldswhich vary periodically along the sensor head with a period T₃ /3. Thesignificance of this point will become apparent later.

FIGS. 7a to 7d illustrate the way in which the multi-turn pick-upwindings 133 and 135 are formed from a single conductor. The parts ofthe windings on the top layer of the printed circuit board arerepresented by full lines and those on the bottom layer are representedby broken lines. As shown in FIG. 7a pick-up winding 133 extends fromvia hole 1 in a generally sinusoidal manner to via hole 44 at the otherend of the sensor head. Similarly, pick-up winding 135 extends from viahole 42 in a generally sinusoidal manner along the length of the sensorhead to via hole 46. As shown, the period of each winding 133 and 135 isT_(s) and winding 135 is spatially shifted by quarter of a period, i.e.T_(s) /4, relative to winding 133.

As shown in FIG. 7b, pick-up winding 133 extends back along the sensorhead from via hole 44 again following the same generally sinusoidal pathback to via hole 21 which is a connection point for connecting pick-upwinding 133 to the excitation and processing circuitry (not shown).Pick-up winding 133 continues from the other connection point at via 20along the sensor head in the same generally sinusoidal manner to via 43.

Similarly, pick-up winding 135 extends from via 44 back along the sensorhead following the same generally sinusoidal pattern until it reachesthe connection point at via 23. Pick-up winding 135 continues from theother connection point at via 22 along the sensor head in the samesinusoidal manner to via 41.

As shown in FIG. 7c, pick-up winding 133 extends from via 43 back alongthe length of the sensor head, again in the same generally sinusoidalmanner to via 40'. Similarly pick-up winding 135 continues from via 41and extends back along the sensor head to via 45, again in the samegenerally sinusoidal manner. As shown in FIG. 7d, the pick-up windings133 and 135 extend from via holes 40' and 45 respectively back along thelength of the sensor head to the starting points at via holes 1 and 42respectively, as shown in FIG. 7a.

Essentially, the inventor has added an extra loop to the end of one ofthe windings in order to make their mid-points coincide. He has alsoadded a second conductor to some of the loops so that the sensor head isstill relatively immune to background electromagnetic interference andso that each winding encloses approximately the same area. To explainfurther, FIG. 8a schematically shows two sensor windings 133' and 135'having different mid points. Winding 133' has mid point 134 and winding135' has mid point 136. FIG. 8b shows that by adding the additional loop138 (shown in phantom) to the end of winding 133', that the effectivemid point of winding 133' is now at point 136, ie. the same as that forwinding 135'. Therefore, the windings shown in FIG. 8b will be lesssensitive to longitudinal tilt relative to the track. However, by addingthe additional loop 138 to the end of winding 133', it becomes sensitiveto background electromagnetic interference since there are no longer thesame number of loops wound in-each sense.

In order to overcome this imbalance, a second turn of conductor isprovided for some of the loops in the windings 133' and 135'. Inparticular, as shown in FIG. 8c, winding 133' has a second turn ofconductor, represented by the dashed loop 140, for the central loop andsense winding 135' has a second turn of conductor, represented by dashedloops 142, for both of its loops. The additional loops which are addedare wound so that there are equal number of loops wound in each sense,ie. so that there is an equal numbers of loops wound clockwise andanticlockwise. The only difference between the windings 133' and 135'shown in FIG. 8c and those shown in FIGS. 5a and 5b is that in FIG. 8cthe windings extend for only approximately one period, whereas in FIGS.5a and 5b the windings 133 and 135 extend for approximately fiveperiods. The multi-period design offers the advantages of increasedsignal strength and of averaging the signal over a number of periodswhich reduces errors due to defects in the manufacture of the windingsand of the track.

In addition to the positional error in the output signal resulting fromthe tilt or pitch of the sensor head-111 shown in FIG. 1 relative to thetrack 101, another positional error arises in the output signal fromeach pick-up winding 103 and 105 if the sensor head 111 rolls about itslongitudinal axis. This is because the conductors which form the pick-upwindings are provided on two sides of a printed circuit board which hasa finite thickness. Therefore, when the sensor head 111 rolls about itslongitudinal axis, the cross-over points between the forward and returnconduction paths change. This results in a perceived positional changealong the x-axis of the sensor head 111 relative to the track-101.

The inventor has found that this positional error can be reduced by, forexample, changing the phase of the conductive windings on each side ofthe printed circuit board approximately half-way along the sensor head111. This results in the apparent positional change in the left handside of the pick-up windings being opposite in sense to the apparentpositional change in the right hand side. Therefore, the two positionalchanges cancel each other out. This positional error can also beminimised by reducing the thickness of the circuit board.

FIGS. 9a and 9b show the conductive patterns and the required via holeson the top and bottom layers of the printed circuit board respectivelyto achieve this phase shift for the pick-up windings 133 and 135 shownin FIG. 4a. As shown in FIG. 9a, the conductive patterns on theleft-hand side of the sensor head 111 which were on the bottom layer ofthe printed circuit board shown in FIG. 4c, are now on the top layer.Similarly, as shown in FIG. 9b, the conductive patterns on the left-handside of the sensor head 111 which were on the top layer of the printedcircuit board shown in FIG. 4b, are now on the bottom layer. A similarcompensation can be made by, for example, notionally splitting thewindings into four quarters and by changing the phase every quarter.

Due to imperfections in the manufacturing process and the inability toconstruct a device having wires which cross over in the same plane andwhich are insulated from each other, positional errors still arise inthe output signals due to tilt and roll of the sensor head 111 relativeto the track. Some of these positional errors can be compensated for bythe processing circuitry. FIG. 10 is a block diagram illustrating theprocessing circuitry used in the present embodiment. In particular, theprocessing circuitry 151 receives the signals 153 from the pick-upwindings 133 and 135 as inputs, and these are fed to the demodulator 155which outputs demodulated signals 157. The demodulated signals 157 arethen processed by the compensation unit 159 which compensates for someerrors which are inherent in the system. The compensated output signals161 from the compensation unit 159 are then processed by the positioncalculation unit 163 which outputs 165 the position of the sensor head111 relative to the track 101. Some of the corrections that thecompensation unit 159 makes will now be described in more detail.

One form of error in the signals induced in the pick-up windings iscaused by unwanted cross-coupling between the pick-up windings 133 and135 and the excitation winding 137. This error is represented by avoltage offset (as illustrated in FIG. 2b) in the demodulated signals157 output from the demodulator 155. In this embodiment, this offset isdetermined by suitable calibration, and is then subtracted from theoutput demodulated signals 157 by the compensation unit 159.

Another form of error arises when the pick-up windings 133 and 135 haveslightly different sensitivities to the opposing fields generated by theconductive screens 103. The different sensitivities may arise because ofdifferences between the areas enclosed by the two windings 133 and 135.To account for this error, the compensation unit applies a weighting toone of the measured output demodulated signals 157. Typically, if eachpick-up winding has the same number of loops, then the weighting will beapproximately 1±0.5% Additionally, a phase offset in the demodulatedoutput signals may also be required if the two pick-up windings 133 and135 are not exactly separated by T_(s) /4. This offset and weighting canalso be determined through suitable calibration.

Another form of error which arises in the output signals is due tovariations in the separation between the sensor head 111 and the track101. The peak amplitude of the demodulated output signals decreases whenthe separation increases and increases when the separation decreases.Therefore, a suitable approximate correction value can be determined bytaking a weighted value of the inverse of the peak amplitude of theoutput demodulated signals 157. The weighting used also compensates forchanges in the cross coupling interference between the excitation andpick-up windings which will also change as the separation changes.Again, this weighting can be determined by suitable calibration. Sincethe demodulated output signals on the pick-up windings 133 and 135 areof the form Asinθ and Acosθ respectively (where A is the peak amplitudeand θ is dependent upon the position), the peak amplitude (A) can bedetermined by squaring the demodulated output voltage from each pick-upwinding, summing the squared values and taking the square root of theresult. In this embodiment, the compensation unit 159 determines anappropriate correction value and subtracts this from the demodulatedsignals 157.

The inventor has established that without the above correction values,the output position is typically accurate to within 2% of the periodT_(s) when the head gap, i.e. the separation between the sensor head andthe track, is between 1 mm and 2 mm. Whereas, using the correctionvalues described above, over sensor head gaps of between 1 mm and 2 mm,with T_(s) equal to 6 mm, accuracies of about 0.1% T_(s) have beenestablished. Additionally, the spatial resolution of the positionencoder with these corrections has been demonstrated to be of the orderof 0.001% T_(s).

A number of modifications which can be made to the above embodimentswill now be described with reference to FIGS. 11 to 14. FIG. 11illustrates an alternative form of track 101, where the conductivescreens 103 have a rectangular slit 171 in the middle. Preferably, thedimension W₂ shown in FIG. 11 should be greater than the separationbetween the via holes 1-40 and 1'-40' on the sensor head 111 shown inFIG. 4, as this reduces the coupling of the magnetic field generated bythe current density on the horizontal limbs of each screen 103 with thewindings 133 and 135 on the sensor head ill. The width d₃ of the slit171 is preferably equal in size to the separation d₄ between adjacentconductive screens 22 and half the width of the vertical limbs 173 ofthe screens 103, in order to reduce spatial harmonic distortion. Morespecifically, when the conductive screens opposite the sensor head areenergised by a current flowing in the excitation winding, a surfacecurrent density is generated on the screens which generates an opposingmagnetic field which varies periodically with position along the track.

FIG. 12a shows the resultant current surface density flowing in thevertical limbs 173 of two adjacent conductive screens 103, ignoring thecurrent density in the horizontal limbs. Since several adjacentconductive screens 103 are energised together, the opposing magneticfield (H_(y)) generated by the conductive screens 103 will be periodicand, at the surface of the screens 103, will have the form generallyshown in FIG. 12b. It can be shown from a Fourier analysis of the signalshown in FIG. 12b, that this signal contains a fundamental componenthaving a spatial period T_(s) and higher order harmonics, and that bymaking d₃ and d₄ equal to 1/6 of the period T_(s), there is no second orthird harmonic content. The significance of this will now be explained.

The magnetic field generated by the conductive screens decaysexponentially with distance from the surface of the screens. Since thehigher order harmonics have a lower amplitude at the surface of thescreens, these components die off more quickly than, for example, thefundamental component. Therefore, if the sensor head is operated at adistance of approximately 1 mm or 2 mm from the surface of the track,then most of the opposing magnetic field coupling with the pick-upwindings 133 and 135 will be due to the lower order harmonics. However,by providing a rectangular slit 24 in the middle of the conductivescreens with the dimensions described above, only the fundamentalcomponent of the opposing magnetic field will dominate at the surface ofthe sensor head 111.

This embodiment has a manufacturing cost equal to that of the firstembodiment, when using printed circuit board construction techniques.Additionally, the input current to output voltage phase relationshipwill be similar to that of the first embodiment.

FIG. 13a illustrates a preferred form of track 101 where the conductivescreens are replaced by resonant circuits 181. As in the firstembodiment, the repetition period of the resonant circuits 181 is thesame as the spatial period T_(s) of the pick-up windings. By usingresonant circuits 181, it is possible to use a pulse-echo mode ofoperation, whereby short bursts of excitation current are applied at theresonant frequency of the resonators 181, and the processing circuitryprocesses the signals induced in the pick-up windings after theexcitation current has been removed. This mode of operation worksbecause the resonators 181 continue to "ring" for a short period of timeafter the burst of excitation current has been removed. This mode ofoperation eliminates possible cross-coupling between the excitationwindings and the pick-up windings.

As shown in FIG. 13a, each resonant circuit 181 of this embodimentcomprises a coil 183 and a capacitor 185. One end of the coil 183 isconnected to an end of the capacitor 185 and extends away from thecapacitor following an anticlockwise reducing spiral path until itreaches the via hole 187. At via hole 187 the coil 183 passes through tothe other side of the track 101 and continues in an enlarging spiralwinding until it reaches via hole 189, where it passes back through theboard and connects to the other end of the capacitor 185. In thisembodiment the capacitor is a surface mount capacitor, using NPOdielectric with a value of 5.6 nF, which is mounted on one side of theprinted circuit board having a thickness of approximately 0.4 mm. Thedimension W₂ shown in FIG. 13a should be greater than the separationbetween opposing via holes 1-40 and 1'-40' shown in FIG. 4, as thisreduces the unwanted effects of the current flowing in the horizontalconductors of the coils. In this embodiment, the separation betweenopposing via holes is 6 mm and the dimension W₂ is 8 mm. The height H ofthe coil 183 in this embodiment, is 13 mm. As in the embodiment shown inFIG. 11, the widths d₃ and d₄ are preferably equal to approximately 1/6of the period T_(s), as this reduces the unwanted 2nd and 3rd spatialharmonics of the magnetic field produced by the resonators 181.

FIG. 13b, the top copper layer, shows the reducing anticlockwise spiralwinding of coil 183. FIG. 13c, the bottom copper layer (as viewed fromthe top copper layer), shows the enlarging anticlockwise spiral windingof coil 183.

As is well known in the art, the Q of a resonant circuit is dependentupon the area of copper which forms the coil, for a given copper foilthickness. The use of a multi-turn coil on both sides of he track 101 isconsistent with maintaining a high Q and reasonable impedance levels.Preferably, high stability capacitors are used with the multi-turn coils183 in the resonator design. Embodiments with one or two turns areenvisaged, but capacitors having the required values and stability arenot manufactured presently.

In the resonator embodiment, with a typical drive current ofapproximately 100 mA, at a frequency of approximately 1 MHz, and with aseparation between the sensor head 111 and the track 101 ofapproximately 1 mm, the voltage induced in the pick-up windings will beapproximately 100 mV rms. However, as the separation increases, theoutput voltage reduces exponentially. For example, at a separation of 2mm the voltage output from the pick-up windings is approximately 30 mVrms.

As mentioned above, the advantage of using resonant circuits as thepassive elements on the track 101, is that the system can be operated ina pulse-echo mode of operation. However, since the impedance of theresonant circuits 181 at resonance is purely resistive, there is a welldefined phase relationship between the excitation current and thevoltage induced in the pick-up windings. Therefore, even if theexcitation current is continuously applied to the excitation winding,the processing circuitry (not shown) will be able to differentiatebetween the signals induced in the pick-up windings by the resonatorsfrom the signal induced in the pick-up windings by the excitationwinding. In particular, when resonant circuits are used, the phase ofthe synchronous detector is set to give the minimum cross-couplingoffset error.

FIG. 14 illustrates an alternative embodiment, where the track 101carries a plurality of short circuit coils 183 in place of theconductive screens 103. Again, the repetition period of the shortcircuit coils 183 is set to be equal to the spatial period T_(s) of thepick-up windings. This embodiment operates in a similar manner to theembodiment described above with reference to FIG. 11. This embodiment isslightly cheaper to implement than the resonant circuit embodiment asthere is no capacitor. The dimensions of the coils 183 in thisembodiment are the same as those of the coils shown in FIG. 13a.

Although sinusoidal and hexagonal shaped pick-up windings have beenshown in the drawings, alternative geometries or patterns of conductorscould be used. For example, square-wave or triangular-wave windings andany other three piecewise linear approximations.

Although two-phase quadrature pick-up windings have been employed on thesensor head 111, in the above embodiments, a sensor head employingthree, four or any number of suitably shifted pick-up windings could beused. For example, three pick-up windings could be provided on thesensor head, each spatially separated from the other by T_(s) /6.

The operating frequency of the encoder is mainly determined by physicalsize and the required circuit impedances. Typically, the operatingfrequency ranges from 10 KHz to 10 MHz, with 300 KHz being optimal forthe six millimeter pitch pick-up windings and conductive screens. Theoptimum operating frequency when resonators are used is dependent uponthe resonator Q, but will typically be about 1 MHz. In the resonatordesign, the required circuit impedance can be obtained by using seriesor parallel connected resonating capacitors if required. Alternatively,impedance electrical transformers can be utilised which have the addedadvantages of introducing galvanic isolation, suppressing common modeinterference signals and improving the power efficiency of the sensor.

The inventor believes that by using these correcting techniques, it ispossible to scale the device over a very large range of pick-up windingperiod T_(s). For example, it is envisaged that the sensor head could beimplemented on silicon. In this case, it would be possible to implementthe entire sensor head including the processing circuitry, on a singleintegrated circuit chip.

Furthermore, it is possible to shield the position encoder system fromsurrounding electromagnetic interference, thereby allowing the device tobe used in electromagnetically hostile environments. In addition, thesystem is not adversely affected if a steel backing plate is providedbehind the track 101 and/or if a thin stainless steel (i.e.non-magnetic) layer is placed over the track. However, when such astainless steel cover is used, the operating frequency must besufficiently high to make the stainless steel seem transparent to thegenerated magnetic fields. Therefore, the system can be used in a widevariety of applications, including high accuracy industrialapplications, such as machine tool position sensing.

It is also envisaged that the track 101 could be formed into a circularring, thereby providing a rotary position sensor.

The present invention is not intended to be limited by the exemplaryembodiments described above, and various other modifications andembodiments will be apparent to those skilled in the art.

What is claimed is:
 1. A position detector including:first and secondmembers mounted for relative movement in a measurement direction; saidfirst member comprising a sensor having first and second sensing means,each comprising a plurality of loops of conductor, arranged insuccession in said measurement direction, wherein i) the loops of therespective sensing means extend along said path and are connected inseries with each other and arranged so that EMFs induced in adjacentloops by a common background alternating magnetic field oppose eachother; ii) the loops of said first and second sensing means are offsetfrom each other in the measurement direction; iii) the extent of each ofsaid loops in said measurement direction is substantially the same; andiv) the loops of the respective sensing means are arranged such that themidpoints of the plurality of loops of conductor of the first and secondsensing means, in the measurement direction, substantially coincide;said second member comprising a plurality of equally spaced magneticfield generators each for generating a magnetic field, said magneticfield generators extending along said measurement direction with arepetition period in said measurement direction that is substantiallyequal to twice the extent of said conductor loops in the measurementdirection; and wherein the magnetic fields generated by the magneticfield generators adjacent said sensor induce, in each of said sensingmeans, a signal whose amplitude varies with the position of the sensorrelative to the magnetic field generators in the measurement direction,the amplitude of the signal induced in the first sensing means beingdifferent from the amplitude of the signal induced in the second sensingmeans for a given relative position between the magnetic fieldgenerators and the sensor, due to the offset between the loops of thefirst and second sensing means in the measurement direction.
 2. Adetector according to claim 1, wherein the plurality of loops ofconductor of each sensing means are generally symmetric about atransverse reflection plane which passes through said mid point.
 3. Adetector according to claim 1, wherein the plurality of loops ofconductor of each sensing means are arranged to enclose a similar area.4. A detector according to claim 1, wherein each sensing means comprisesthe same number of series connected loops of conductor, and wherein oneof the plurality of loops of conductor extends over a greater distancein the measurement direction than the other.
 5. A detector according toclaim 4, wherein the loops at each end of the longer plurality of loopsare arranged to have a reduced sensitivity to magnetic field than thesensitivity to magnetic field of the other loops.
 6. A detectoraccording to claim 5, wherein more turns of conductor are provided fordefining said other loops than the number of turns of conductor used fordefining said end loops, thereby causing the sensitivity to magneticfield of said end loops to be reduced relative to the sensitivity tomagnetic field of said other loops.
 7. A detector according to claim 6,wherein two turns of conductor are used-to define said other loops and asingle turn of conductor is used to define said end loops.
 8. A detectoraccording to claim 6, wherein the spacing between the turns of eachconductor is arranged to reduce the sensitivity of each conductor tomagnetic fields which vary periodically in the measurement directionwith a period which is two thirds of the extent of said loops in saidmeasurement direction.
 9. A detector according to claim 1, whereinconnections to each of said plurality of loops of conductor are providedin the vicinity of said mid point.
 10. A detector according to claim 1,wherein the loops of said first and second sensing means have agenerally hexagonal shape.
 11. A detector according to claim 1, whereinthe loops of said first and second sensing means have a generally squareshape.
 12. A detector according to claim 1, wherein the loops of saidfirst and second sensing means are derived from opposed sinusoidalconvolutions of conductor.
 13. A detector according to claim 1, whereinthe loops of each sensing means are carried on a substantially planarsurface.
 14. A detector according to claim 13, wherein the loops of thefirst and second sensing means are electrically separated from eachother, and formed one on top of the other.
 15. A detector according toclaim 1, wherein the loops of said first and second sensing means areoffset from each other in the measurement direction by half the extentof said loops.
 16. A detector according to claim 1, wherein each sensingmeans comprises more than two of said loops which extend successively insaid measurement direction.
 17. A detector according to claim 16,wherein each sensing means comprises approximately ten of said loops.which extend in said measurement direction.
 18. A detector according toclaim 1, wherein the extent of each of said loops is approximately 3 mm.19. A detector according to claim 1, wherein the loops of each sensingmeans are defined by a plurality of interconnected conductive patternson a plurality of layers of a printed circuit board.
 20. A detectoraccording to claim 19, wherein the loops of each sensing means aredefined by two repetitive complementary conductive patterns, eachrepetitive pattern being formed on a respective side of a printedcircuit board.
 21. A detector according to claim 19, wherein the loopsof each sensing means are defined by two repetitive complementaryconductive patterns on two sides of a printed circuit board, and whereineach side of the printed circuit board carries a portion of eachrepetitive pattern.
 22. A detector according to claim 21, wherein eachside of the printed circuit board carries half of each repetitivepattern.
 23. A detector according to claim 1, comprising an excitationcircuit mounted for movement with said sensor relative to said magneticfield generators, which excitation circuit is operable for generating anenergizing signal in the vicinity of said sensor and wherein each ofsaid magnetic field generators is operable to generate a magnetic fieldwhen energized by the energizing signal generated by said excitationcircuit.
 24. A detector according to claim 23, wherein said excitationcircuit comprises a plurality of conductive windings which extend aroundthe periphery of the loops of conductor of said first and second sensingmeans.
 25. A detector according to claim 1, wherein each of said firstand second sensing means are arranged to be substantially insensitive toinduction of signals by an alternating magnetic field of uniformstrength along the measurement direction.
 26. A detector according toclaim 1, wherein each magnetic field generator comprises a conductivescreen.
 27. A detector according to claim 1, wherein each magnetic fieldgenerator comprises a resonant circuit.
 28. A detector according toclaim 27, wherein each resonant circuit comprises a coil and acapacitor.
 29. A detector according to claim 1, wherein each magneticfield generator comprises a short circuit coil.
 30. A detector accordingto claim 29, wherein said short circuit coil comprises a single turn ofconductor.
 31. A detector according to claim 1, wherein the arrangementof the magnetic field generators is such that the combined magneticfield generated by them contains little or no field components whichperiodically vary in the measurement direction with a period which istwo thirds that of the extent of the loops of conductor.
 32. A detectoraccording to claim 1, comprising processing circuitry which is operablefor processing the signals induced in said first and second sensingmeans, and which is operable for providing an output indicative of therelative position of the sensor and the magnetic field generators.
 33. Adetector according to claim 32, wherein said processing circuitrycomprises a synchronous detector which is operable for demodulating thesignals induced in said first and second sensing means.
 34. A detectoraccording to claim 33, wherein the synchronous detector is arranged tominimise any interference caused by the excitation signal.
 35. Adetector according to claim 33, wherein the synchronous detector isarranged to provide the maximum output signal levels.
 36. A detectoraccording to claim 33, wherein said processing circuitry is arranged tosubtract an offset from the demodulated signals in order to compensatefor interference caused by the excitation signal.
 37. A detectoraccording to claim 33, wherein the processing circuitry is arranged toapply a weighting to the demodulated output signal from each of saidfirst and second conductors to compensate for different sensitivities tomagnetic field of each of said first and second sensing means.
 38. Adetector according to claim 33, wherein said processing circuitry isarranged to provide a phase offset in the demodulated output signalsfrom said first and second sensing means, in order to compensate forerrors which arise because the loops of conductors of the first andsecond sensing means are not offset in said measurement direction by therequired amount.
 39. A detector according to claim 33, wherein theprocessing circuitry is arranged to subtract a second offset from thedemodulated output signals from said first and second conductors, inorder to compensate for variations in separation between the sensor andthe plurality of magnetic field generators.
 40. A detector according toclaim 39, wherein the second offset value is dependent upon the inverseof the peak amplitude of the measured demodulated output signals fromthe first and second sensing means.
 41. A detector according to claim33, wherein said processing circuitry is arranged to determine a ratioof the demodulated signals from said first and second sensing means toprovide said output indicative of the relative position of the sensorand the magnetic field generators.
 42. A sensor for use in a positiondetector according to claim 1, comprising:first and second sensingmeans, each comprising a plurality of loops of conductor extending in ameasurement direction, wherein i) the loops of the respective sensingmeans are connected in series to each other and arranged so that any emfinduced in one of said loops by an alternating magnetic field of uniformstrength is opposed by the emf induced in an adjacent connected loop bythe same magnetic field; ii) the loops of said first and second sensingmeans are offset from each other in the measurement direction; iii) theextent of each of said loops in said measurement direction issubstantially the same; and iv) the loops of the respective sensingmeans are arranged such that the midpoints of the plurality of loops ofconductor of the first and second sensing means, in the measurementdirection, substantially coincide.
 43. Use of a sensor according toclaim 42, in an apparatus for indicating the position of first andsecond relatively movable members.
 44. (Amended) A method of determiningthe relative position of first and second relatively movable membersusing a detector according to claim 1, comprising the steps of:providingsaid sensor on said first member and said plurality of magnetic fieldgenerators on said second member; generating magnetic fields frommagnetic field generators in the vicinity of said sensor; and detectingthe output signals produced in response thereto in said sensor andderiving therefrom the relative positions of said first-and secondrelatively movable members.
 45. A method according to claim 44, whereinsaid second member is fixed.
 46. A method according to claim 44, whereinthe magnetic field generated by said magnetic field generators is analternating magnetic field having a frequency in the range of 10 KHz to10 MHz.
 47. A method according to claim 44, wherein said magnetic fieldgenerators generate a magnetic field during a first time interval andwherein said output signals are detected during a subsequent second timeinterval after said first time interval.
 48. A position detectorcomprising:first and second members mounted for relative movement in ameasurement direction; said first member comprising an excitationcircuit and a sensor, the sensor having first and second sensing means,each comprising a plurality of loops of conductor arranged in successionin said measurement direction, wherein (i) the loops of the respectivesensing means extend along said path and are connected in series witheach other and are arranged so that EMFS induced in adjacent loops by acommon background alternating magnetic field oppose each other; (ii) theloops of said first and second sensing means are offset from each otherin the measurement direction; (iii) the extent of each of said loops insaid measurement direction is substantially the same; (iv) the loops ofthe respective sensing means are arranged such that the midpoints of theplurality of loops of conductor of the first and second sensing means,in the measurement direction, substantially coincide; and (v) saidexcitation circuit is operable to generate an energising signal in thevicinity of said sensor; said second member comprising a plurality ofequally spaced magnetic field generators extending along saidmeasurement direction, wherein (i) the repetition period of saidmagnetic field generators in said measurement direction is substantiallyequal to twice the extent of said conductor loops in the measurementdirection; and (ii) each of said magnetic field generators is operableto generate a magnetic field when energised by the energising signalgenerated by said excitation circuit, which magnetic field is operableto induce, in each of said sensing means, a signal which varies with theposition of the sensor relative to the magnetic field generators in themeasurement direction, from which the relative position of said firstand second members can be determined.
 49. A position detectorincluding:first and second members mounted for relative movement in ameasurement direction; said first member comprising a sensor havingfirst and second sensor windings, each comprising a plurality of loopsof conductor arranged in succession in said measurement direction,wherein i) the loops of the respective sensor windings extend along saidpath and are connected in series with each other and arranged so thatEMFs induced in adjacent loops by a common background alternatingmagnetic field oppose each other; ii) the loops of said first and secondsensor windings are offset from each other in the measurement direction;iii) the extent of each of said loops in said measurement direction issubstantially the same; and iv) the loops of the respective sensorwindings are arranged such that the midpoints of the plurality of loopsof conductor of the first and second sensor windings, in the measurementdirection, substantially coincide; said second member comprising aplurality of equally spaced magnetic field generators, said magneticfield generators extending along said measurement direction with arepetition period, in said measurement direction, that is substantiallyequal to twice the extent of said conductor loops in the measurementdirection; and wherein the arrangement is such that magnetic fieldsgenerated by magnetic field generators in the vicinity of said sensorinduce, in each of said sensor windings, a signal that varies with theposition of the sensor relative to the magnetic field generators, fromwhich the relative position of said first and second members can bedetermined.
 50. A position detector comprising:first and secondrelatively movable members: one of said members includes a sensor headand the other of said members includes a plurality of spaced magneticfield generators; said sensor head including loops of conductor inrespective first and second sensing circuits arranged such thatmidpoints of the plurality of loops of conductor of said first andsecond sensing circuits, in a measurement direction, substantiallycoincide, thereby providing a detector which is less sensitive tolongitudinal tilt of the sensor head member relative to the othermember.